CH^ETOPODA. 325 



Mollusca the origin of the mesoblast in Planorbis, p. 227, is very similar to 

 that in Lumbricus. 



Hatschek has shewn that in Polygordius the mesoblast arises in funda- 

 mentally the same way as in the Oligochaeta. 



Besides the mesoblast which arises from the mesoblastic bands, there 

 is evidence of the existence of further mesoblast in the larvae of many 

 Polychaeta in the form of muscular fibres which traverse the space between 

 the body wall and the wall of the enteric cavity prior to the formation 

 of the permanent body cavity. These fibres have already been described 

 in the embryo of Serpula, and are probably represented by stellate cells 

 in the cephalic region (prae-oral lobe) of the Oligochaeta. These cells are 

 probably of the same nature as the amoeboid cells in the larvae of Echino- 

 dermata, some Mollusca and other types. 



The Larval form. 



True larval forms are not found in the Oligochaeta where the 

 development is abbreviated. They occur however in the ma- 

 jority of the marine Polychaeta. 



They present a great variety of characters with variously 

 arranged ciliated bands. Most of these forms can be more or 

 less satisfactorily derived from a larval form, like that of Serpula 

 (fig. 139 B) or Polygordius (fig. 142); and the constant recur- 

 rence of this form amongst the Chaetopoda, combined with the 

 fact that it presents many points of resemblance to the larval 

 forms of many Rotifers, Molluscs, and Gephyreans, seems to 

 point to its being a primitive ancestral form for all these 

 groups. 



The important characters of this larval form are (i) the 

 division of the body into a large prae-oral lobe and a relatively 

 small post-oral region containing the greater part of the alimen- 

 tary tract ; (2) the presence of a curved alimentary canal 

 divided into stomodaeum (oesophagus), stomach and intestine, 

 and opening by a ventrally placed mouth, and an anus near the 

 hind end of the body. To these may be added the frequent 

 presence of (i) a ganglion at the apex of the prae-oral lobe, 

 (2) a large cavity between the wall of the gut and the skin, 

 which is the remnant of the segmentation cavity, and is usually 

 traversed by muscular strands, of which one connecting the apex 

 of the prae-oral lobe and the stomach or oesophagus is very 

 commonly present (fig. 142). 



The arrangement of the ciliated bands presents great varia- 



